Thermoelectric energy conversion:

conversion of heat into electrical energy or vice versa

Temperature gradient creates a potential difference; connecting the 2 ends via an electrical conductor results in a current flow.

Thermoelectrics are materials which are capable of converting heat into electrical energy and vice versa. This fascinating phenomenon is nowadays commercially used in power generators (e.g., in the telecommunication industry, or in spacecrafts), food refrigerators, air conditioning, cryotherapy, pacemakers, and sensors (e.g. thermocouples). The automobile industry is eager to use this technique, e.g. for environmentally harmless air conditioning or as a power source for the radio or headlights, driven by the exhaust heat. The applications are to date limited due to the somewhat low efficiency η (ca. 5 - 10 %):

Typical values might be T_H = 800 °C, T_C = 400 °C, ZT = 1, yielding a theoretical η = 7.6 %.

Thermoelectrics are evaluated based on their thermoelectric figure-of-merit ZT, which is close to 1 in the materials commercially used. The Seebeck coefficient S and the thermal conductivity can be measured in our group using the commercial ZEM-3 M-8 (ULVAC-RIKO).

An increase of ZT by a factor of two or more would be necessary to become competitive to Freon compressors as used in conventional refrigerators.
The best thermoelectrics exhibit an intermediate charge carrier concentration (e.g., small band-gap semiconductors), a high mobility of the charge carriers (achieved by using elements with similar electronegativities) and low thermal conductivity, which can be reached by using heavy elements, mixed occupancies, rattling of atoms, and low-symmetry structures.

To improve on these characteristics is the ultimate goal in our research group.

S optimization

Optimization of the Seebeck coefficient by changing the charge carrier concentration (own research)

ZT optimization

Temperature dependence of the thermoelectric figure-of-merit of new materials (own research)

Recommended references:

H. Kleinke, Chem. Mater. 22, 604 (2010):
a review covering advanced thermoelectrics for power generation.
X. Cheng, N. Farahi, H. Kleinke, JOM 68, 2680 (2016):
a review covering environmentally benign Mg₂Si materials for power generation.
N. Farahi, S. Prabhudev, G. A. Botton, J. R. Salvador, H. Kleinke, ACS Appl. Mater. Interfaces, doi: 10.1021/acsami.6b12297:
nano- and microstructure engineering enhances ZT to 1.4 for Bi-doped Mg₂(Si,Sn).
Q. Guo, A. Assoud, H. Kleinke, Adv. Energy Mater. 4, 1400348 (2014):
Tl_10-δSn_δTe₆ and Tl_10-δPb_δTe₆ exceed ZT = 1.
N. Farahi, H. Kleinke, et al., Dalton Trans., 43, 14983 (2014):
properties of n-doped Mg₂Si; the dopant replaces Si.
N. D. Y. Truong, H. Kleinke, F. Gascoin, Dalton Trans., 43, 15092 (2014):
adding carbon nanotubes to MnSi_1.75 enhances ZT.
S. Derakhshan, K. M. Kleinke, E. Dashjav, H. Kleinke, Chem. Commun., 2428 (2004):
creating the first nonmetallic early transition metal antimonide.
E. Dashjav, A. Szczepenowska, H. Kleinke, J. Mater. Chem. 12, 345 (2002):
    properties of p-type thermoelectrics Mo₃Sb_5+δTe_2-δ;
   F. Gascoin, J. Rasmussen, G. J. Snyder, J. Alloys Compd. 427, 324 (2007):
    Mo₃Sb_5.4Te_1.6 reaches ZT = 0.80;
   N. Soheilnia, H. Xu, H. Zhang, T. M. Tritt, I. Swainson, H. Kleinke, Chem. Mater. 19, 4063 (2007):
    properties of n-type thermoelectrics Re₃Ge_δAs_7-δ;
   H. Xu, K. M. Kleinke, T. Holgate, H. Zhang, Z. Su, T. M. Tritt, H. Kleinke, J. Appl. Phys. 105, 053703 (2010):
    Ni_0.06Mo₃Sb_5.4Te_1.6 reaches ZT = 0.93;
   N. Nandihalli, Q. Guo, S. Gorsse, A. U. Khan, T. Mori, H. Kleinke, Eur. J. Inorg. Chem., 853 (2016):
    adding SiC nanoparticles and spark-plasma-sintering enhances ZT.
E. J. Winder, A. B. Ellis, G. C. Lisensky, J. Chem. Educ. 73, 940 (1996):
thermoelectrics in the classroom, for students.
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